1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for preventing a leakage to atmosphere of a compressed medium by a stack rotor.
2. Discussion of the Background
Turbo-machines are used extensively in the oil and gas industry for performing fluid compression, transformation of electrical energy into mechanical energy, fluid liquefaction, etc. One such machine is a compressor. Modern compressors include plural stages (e.g., plural impellers connected in series) that are configured to compress a medium, each stage compressing the medium in a certain pressure range. A single rotor (made for example, as a single solid piece of metal) may be used to hold the plural impellers. However, advanced compressors use a more complex rotor that has a couple of components in order to achieve higher pressure ratio and delivered head.
With regard to FIG. 1, such a complex rotor 10 (which is disclosed in U.S. Pat. No. 3,749,516, the entire disclosure of which is incorporated herein by reference) may include stubs 12 and 14 that sandwich between them plural impellers 16, 18, 20, and 22. In order to hold tight the impellers 16, 18, 20, and 22 one to the other, a thru-bolt 30 is threaded and attached (screwed) at both ends into the stubs 12 and 14.
Stub 12 is attached to first impeller 16 via a longitudinal pin 24 while stub 14 is attached to the impeller 22 via a key 26 along a radial direction. The pin 24 and key 33 provide a driving connection between the impeller assembly and the stubs 12 and 14. It is noted that the thru-bolt is first screwed into the stub 12, then impellers 16, 18, 20, and 22 are added to the thru-bolt 30, and finally the stub 14 is screwed into the thru-bolt 30. For this reason, the pins 24 extend along an axial direction of the rotor and the keys 26 extend along a radial direction of the rotor. However, such a rotor may be difficult to compress, i.e., to connect impeller 22 to stub 14 and apply an appropriate load as an exact alignment between impeller 22 and stub 14 is needed for inserting key 26.
Other existing rotors have a hollow rotor through which the thru-bolt extends fully under the bearing and seal zone and have therefore an extremity accessible from outside of rotor. In order to apply the necessary load to the thru-bolt, one end of the thru-bolt is threaded into the rotor while the other end communicates with an opening in the rotor. This arrangement creates an additional potential leaking path for the compressed medium, between the thru-bolt and the hollow rotor, which is a potential hazard especially if the compressed medium is different from air (e.g., asphyxiating, toxic, explosive or a combination of all). The potential leaking path appears as the medium compressed by the compressor is at high pressure and thus, part of the compressed medium may escape by the rotor towards an area of low pressure. Systems to seal such type of configuration can be provided but they will nevertheless have the potential to fail.
Accordingly, it would be desirable to provide systems and methods that provide the operator of the machine with easy access to the thru-bolt and also does not leak between the thru-bolt and the rotor or other parts of the machine.